The present invention relates to a method of operating a wireless local area network.
With a view toward obviating the need for wired cabling connections between stations in local area networks (LANs), wireless local area networks have been developed and are now commercially available. However, the coverage area of wireless LANs is restricted especially in an indoor environment, due to the presence of structural features such as walls and floors in buildings, for example. Also it may be desirable for stations in a wireless LAN to communicate with remote facilities or resources such as servers. Therefore, it has been proposed to connect the wireless LAN via interface devices to a backbone LAN such as a wired LAN, which can be connected to the remote facilities and/or other wireless LANs.
Furthermore, where a mobile wireless station, such as a portable data processing unit provided with a wireless transceiver, for example, moves around it may move in and out of the range of various interface devices connected to the backbone LAN. It is therefore necessary for the mobile station to hand over communication from one interface device to another as it moves around.
European Patent Application No. 0 483 544 discloses a wireless communications network including a plurality of mobile data stations in bidirectional communication with header stations which are connected into a wired LAN and which define communication cells. If a mobile data station crosses over from one cell to another, the mobile station determines a new header station for communications access to the wired LAN. The header stations periodically transmit broadcast messages identifying the header station and the loading factors at the header station. Using these messages a mobile station will determine all header stations that are potential owners of the mobile station and maintain in memory a list of potential owner stations. If the current owner station remains a potential owner, and the signal strength of the current owner station is acceptable, then the current owner is retained. Otherwise, the mobile station determines a new header station based on the loading factors of the potential owner stations, and, if the loading factors are equal, the signal strengths of the signals received at the mobile station. This procedure has the disadvantages of using memory space in the mobile station to maintain in memory the list of potential owner stations, and the need to constantly monitor the messages periodically transmitted by the base stations.
International Patent Application No. WO 92/19059 discloses a system which includes a cable-based network to which are attached controllers which maintain a portable device in communication with the network for data transfer. The portable unit transmits polling packets at regular intervals. Normally, a response packet is received from the current controller. If no response packet is received for a predetermined number of attempts the portable unit initiates a procedure for registering with a new controller. This procedure involves transmitting a packet from the portable unit requesting a response from any controllers, transmitting a response packet from each controller receiving the request, selecting one of the responding controllers at the portable unit according to criteria that enhance data transfer, and transmitting from the portable unit a packet identifying the selected controller to complete registration. This procedure has the disadvantage of the need for the portable unit to transmit a request packet to initiate the handover procedure.